Technological Field
The present disclosure relates to improvements in or relating to imaging sensors, and is more particularly, although not exclusively, concerned with in-pixel amplification.
Description of the Related Technology
Imaging sensors are implemented using CMOS technology to reduce noise so as to produce a satisfactory resolution of the image being captured in each imaging frame. Generally, a typical imaging sensor comprises an array of pixels, each pixel producing an output signal (or pixel output) corresponding to the light level relating to a particular portion of the image being captured incident on a photodiode of the pixel during an integration period for that photodiode. Pixel outputs must be read out with low noise in order to be able to provide the desired resolution of the overall captured image. Ideally, the pixel output should be amplified as soon as possible. This means that the output should be amplified very close to the pixel, or preferably inside the pixel itself. However, if the pixel is small, there may not be sufficient space to accommodate an amplifier.
When amplifying the pixel output very close to the pixel with a gain greater than one, traditionally NMOS and PMOS devices are used. However, for PMOS devices, an N-well is needed which has the disadvantage that photo-electrons are drawn away from the photodiode within the pixel resulting in degraded quantum efficiency. In addition, the requirement for an N-well also limits the fill factor of the pixel.
Moreover, for global shutter operation in an imaging sensor, all pixel outputs need to be stored at the same time to avoid motion effects from a rolling shutter. Typically, the pixel output must be stored inside the pixel to be able to do this as all pixels in a row in an imaging sensor array cannot be read out at the same time. The pixel may be small, and therefore only a small storage capacitor (capacitor with a low capacitance value) can typically be used inside the pixel itself. Such small storage capacitors generate large kT/C noise (as the capacitance value C is small), resulting in a low pixel output signal and poor signal-to-noise performance.